Definition:A turbine may be defined as, “the turbine is a prime mover in which a rotary motion is obtainedby centrifugal force brought into action by changing the direction of a jet or a fluid escapingfrom a nozzle at high velocity.“ Principle of operationSteam turbine is a prime mover in which rotary motion is obtained by the gradual change ofmomentum of the steam. • Steam is passed through nozzle or fixed blades • The heat drop takes place, • Increasing the velocity of steam. • Causes the direction of the steam to be changed.Due to this change of momentum motive force is exerted on the moving blades and power isobtained. Applications:In large sizes, it is used for driving electric generators and ship propellers.In smaller sizes it is adaptable for driving pumps, fans, compressors etc.In general the turbine is well adapted for work which requires high rotation and constant speeds even with widely fluctuating loads.Dr.Khaled S. Al-Zahrani 2
Classification of steam turbinesThere are several ways in which the steam turbines may be classified. The most important andcommon division being with respect to the action of steam as: 1. Impulse 2. Reaction Impulse turbineAn impulse turbine, as the name indicates is a turbine, which runs by the impulse of steam jet.In an impulse design of steam turbine, there is an attempt to attain no expansion of steam withinthe moving blades and as such its pressure remains constant while passing over the blades. Theexpansion of steam, however takes place in a set of stationary nozzle which lowers its pressurebut impact a high velocity to it. The velocity attained Disadvantages:1. velocity of the wheel is too, high (25000 to 30000 r p m.) for practical purposes.2. loss of kinetic energy. In an actual turbine this loss is 10% to 12%. Reaction turbine In case of reaction turbine, the steam expands as it flows over the blades which, therefore act as nozzles. It consists of large number of stages, each consisting of fixed and moving blades. The heat drop takes place throughout in both fixed and moving blades.
Impulse Vs. Impulse Reaction Turbine 1. In impulse turbine the steam completely expands in the nozzle and its pressure remains constant during its flow through the blade passages. In impulse reaction turbine, steam expands partially in the nozzle and further expansion takes place in the rotor blades. 2. The impulse turbine blades have symmetrical profile where the reaction turbine blades have aero foil section 3. The pressure on both ends of the moving blade of an impulse turbine is same whereas different pressure exists on two ends of the moving blade of a reaction turbine. 4. The blade efficiency curve for the reaction turbine is more flat compared with the efficiency curve of impulse turbine. 5. The steam velocity in a reaction turbine is not very high and hence the speed of the turbine is relatively low. 6. The number of stages required for reaction turbine are more compared with impulse turbine for the same power developed as the Pressure drop in each stage is small.Dr.Khaled S. Al-Zahrani 5
* 1. Nozzle loss 2. Blade friction loss 3. Wheel friction loss 4. Mechanical friction loss 5. Leakage loss 6. Governing lossDr.Khaled S. Al-Zahrani 6
* The function of a governor is to: regulate the supply, of steam to the turbine so as to maintain as far as possible a constant mean speed under varying load conditions. There are four methods of governing used in steam turbines. 1. Throttle governing. 2. Nozzle control governing. 3. By-pass governing. 4. Combination of throttle, and nozzle gov. or throttle and by-pass governing.Dr.Khaled S. Al-Zahrani 7
* 1-2: Reversible adiabatic (Isentropic) compression process in the pump. 2-3: Heat adding at constant pressure 3-4: Reversible adiabatic (Isentropic) expansion in the steam turbine 4-1: Heat rejection at constant pressureDr.Khaled S. Al-Zahrani 8
* Advantages of Reheating of steam 1. It increases the work done through the turbine. 2. It increases the efficiency of the turbine. 3. It reduces the water on the blades, because of low moisture contents in low pressure stages of the turbine.Dr.Khaled S. Al-Zahrani 9